A publicly known electric motor-driven booster is disclosed, for example, in Patent Literature 1. The electric motor-driven booster disclosed in Patent Literature 1 operates as follows. According to an amount of driver's operation of a brake pedal, a controller controls an electric motor. Rotary motion of the electric motor is converted into a rectilinear motion through a ball-screw mechanism, which is a rotation-rectilinear motion conversion mechanism. With the rectilinear motion, the ball-screw mechanism propels a piston in a master cylinder, and thus the master cylinder generates a brake hydraulic pressure. Further, in the electric motor-driven booster, the rotational force of the electric motor is transmitted to the ball-screw mechanism through a transmission mechanism comprising a belt and pulleys.